On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 521 
tooth of the lower jaw. It is situated just within the central part of the tooth of 
the upper jaw. The palatal surface of the tooth of the lower jaw is characterized 
by considerable lateral convexity ; and is produced in front to form the beak-like pro- 
minence. Inside each lateral margin, there isa deep sulcus, the palatal and vertical 
portions uniting to form an acuminate and well-developed cutting-edge. The 
palatal surface of the upper tooth is as nearly as. possible the counterpart of the 
lower, so that the two surfaces correspond and fit to each other when closed. 
The interior of the tooth is hollow, the enamelled osseous part forming a thin 
‘shell-like covering to the cartilaginous jaws. There is no portion which can be 
distinguished as the root or base of the tooth, the attachment to the jaw apparently 
being entirely on the inside. 
In searching for the zoological relationship of Pristodus, a striking and most 
peculiar resemblance is at once observed between it and some of the Gymnodont 
eroup of the Plectognath group of fishes at present existing. These fishes are 
characterized by the possession of jaws which in some instances consist of a single 
undivided dental plate to each jaw. In others, the dental arrangement consists 
of two upper and two lower dental plates, divided by a mesial suture. The first 
group is the one to which the Mountain Limestone fossils bear the greatest resem- 
blance. The existing group comprises about seventeen species, which are frequently 
found in the Tropical Atlantic and Pacific seas. A common example is Diodon 
maculatus, which may be taken for comparison with the fossil forms. It is 
unnecessary for our present purpose to consider more than the dental arrangement. 
In an example six inches in length from the snout to the base of the tail, the oral 
aperture is eight-tenths of an inch in width, and both the upper and lower jaw is 
provided with a single semicircular dental plate, extending backwards into the 
mouth, in the form of a palate. The front edge is slightly produced and somewhat 
beak-like, the teeth extending upwards and downwards respectively, encircling 
the outer aspect of the jaw and forming in section two sides of a triangle, the third 
side being occupied by the soft internal muscular or cartilaginous attachment for 
the tooth. This dental arrangement is admirably adapted to break up masses of 
coral, or the hard shells of molluscs and crustaceans on which the fishes feed. 
The upper dental plate extends slightly beyond, and overlaps the lower one ; it is 
slightly more produced or pointed in the centre. Posteriorly, the upper and lower 
jaws are strongly united by ligaments and muscles, the upper extending over and 
embracing the lower. 
The genus Diodon occurs in a fossil state in the tertiary limestone of Monte 
Bolea and Licata, and from Monte Postale, Enneodon, a distinct genus has been 
found and described. 
In many respects the fossil teeth from the Mountain Limestone of Yorkshire, 
bear considerable resemblance to those of Diodon. In the general form of the 
palatal interior, combined with the semicircular external, trenchant edge of the 
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